As of 1415 GMT, shares in Petrobras were up 3.05% at 81.20 Brazilian reals ($45.88), while the benchmark Ibovespa stocks index was down 0.49%.

In its fourth-quarter earnings report, released Thursday, British Gas Group PLC (BG.LN) estimated reserves at the offshore Brazilian Tupi site at between 12 billion and 30 billion barrels, up from its previous estimate of 1.7 billion to 10 billion.

In a regulatory filing Thursday, Galp endorsed the new British Gas estimate.

A spokesman for Petrobras told Dow Jones Newswires the company had no immediate comment on the reports from British Gas and Galp.

The standing Petrobras estimate for the Tupi reserve falls within a range of 5 billion to 8 billion barrels.

Analysts for the Brascan brokerage, located in Sao Paulo, called the British Gas statement "positive" for Petrobras but noted that even the original news of the Tupi find last year has not been fully priced into Petrobras shares.

In a research note, Brascan put the fair price for Petrobras' domestic shares at BRL107.35.